1. Field of Invention
This invention relates to semiconductor lasers, and particularly to stack-type semiconductor laser devices.
2. Description of Prior Art
As used here, the term “semiconductor laser” or “diode laser” means edge-emitting diode lasers. Edge-emitting diode lasers have a horizontal cavity and emit light beams in a direction parallel to the wafer's plane or an active region of the wafer. Most semiconductor lasers in use are edge-emitting and are used for laser printers, fiberoptic telecommunication systems, and optical data storage devices.
A diode emits an output light beam with a spatial power distribution. The output of a single transverse mode diode, or a single-mode diode, has a single lobe in its spatial power distribution, while a multimode output has multiple lobes. Single-mode diode lasers are desirable in many fields, especially in fiberoptic telecommunications and optical data storage. However one of the weaknesses of single-mode diode lasers is that they have relatively low output power.
The importance of tunable wavelength diode lasers has grown dramatically in fiberoptic telecommunications in recent years. Tunable lasers have three important specifications, i.e., output power, wavelength tuning range, and tuning speed. A typical high output power is about 20 milliwatts. A typical wide tuning range is about 40 nanometers, e.g., the diode can emit light from 1525 to 1565 nm. A fast tuning speed is in a range of several nanoseconds. Among various tunable lasers, distributed feedback (DFB) lasers offer high power, but suffer a narrow tuning range and a slow tuning speed. Three-section distributed Bragg reflector (DBR) lasers have a fast tuning speed and moderate power, but a narrow tuning range. Sampled grating DBR (SG-DBR) lasers and superstructure grating DBR (SSG-DBR) lasers have a wide tuning range and a fast tuning speed, but low output power. Conventional tunable external cavity diode lasers benefit from high power and a wide tuning range, but suffer a slow tuning speed. Thus current tunable lasers can't satisfy the three specifications: high power, wide tuning range, and fast tuning speed.
One approach to overcome limitations on the single-mode output power and tunable laser involves stacking two thin-clad laser diodes to form a stack-type diode laser device and coupling the diodes, as is disclosed in my above regular U.S. patent application. The thin-clad diodes emit two beams which are substantially parallel and proximate such that they can simultaneously feed a fiberoptic system designed for a single beam. When the diodes are coupled in phase, their outputs can be combined to increase single-mode power, or novel tunable lasers can be created to meet the three specifications. However, the disclosed coupling mechanisms rely on external feedbacks, which require additional optics. The resulting stack-type device is complicated and bulky.